Process for coating ferrous material and material coated by such process



Y L June 1, 1965 H. B. FoRsLUND ETAL 3,186,867

PROCESS FOR COATING' FERROUS MATERIAL AND MATERIAL COATED BY SUCH PROCESS Filed oct. 12, 1962 /E SURFACE United States Patent() a corporation of New York Filed Oct. 12, 1962, Ser. No. 230,219 4 Claims. (Cl. 117-127) This invention relates'to coatings for ferrous material and, more particularly, Vto a process for coating ferrous material, and the material coated by such process.

In many fields of use and, in particular, in the electrical industry, it is necessary to provide -a coating on ferrous material. This coating desirably performs the functions of insulating, separating and purifying the ferrous material as discussed below. Forexample, in the transformer art, the cores of the Vtransformers are usually formed of a ferrous material, such as, for example, silicon steel, which may be provided with a preferred grain growth orientation to provide optimum electrical and magnetic properties. It has been found necessary to provide a coating on each of the various layers of ferrous material in the core. This coating will perform three separate functions. The first function of the coating is to provide separation of the various turns or layers of the material, for example, when used in cores, to prevent their sticking or welding together during high temperature anneals. A second function is that of aiding in the chemical purification of the ferrous material to develop the desired optimum magnetic characteristics of such material. The third function of the coating is to form on the surface of the ferrous material an insulation which will have suicient electrical strength to provide for the electrical insulation of one layer of ferrous material from the next, for example, during its use as a core in a transformer.

In the present state of the electrical apparatus art, the most widely used coating for the ferrous material which t is used as the magnetic core of the eleotnical apparatus is a coating of magnesium oxide and/ or magnesium hydroxide. These coatings are, in general, applied to the ferrous material in the form of a suspension of magnesif um hydroxide in water. The suspension comprises a quantity of magnesium oxide in water and is mixed suf- Aerally annealed first to provide a grain growth anneal which develops the optimum magnetic propertiesV of the v silicon steel. This anneal is usually carried out at a tem- 'perature ranging from approximately 950 to 1200 C.

This anneal also aidsrin purifying the steel, aided by the i coating placed on the steel. After the magnetic core has been formed, a stress relief anneal is provided to relieve the stresses which have developed in the silicon steel of ice the magnetic core due to the mechanical working of the steel in formation of such core. These stress relief anneals are generally carried out at a temperature of approximately 840 C. Y f

A more economical method of manufacture has been developed in which the grain growth anneal of the silicon steel is deferred until after the core has been formed. In this method it is then possible to provide both a graingrowth anneal and a stress-relief anneal yat the same time. A suitable temperature of approximately 1150 C. is used, under proper atmospheric conditions, to provide this dual anneal. However, as will be understood, in either instance it is necessary to 'provide an interlaminar separator to prevent the sticking or welding of the turns during the anneal, as well as'to provide the other two functions .asV

hereinbefore noted. A

As will be well understood, Where the grain growth anneal is provided prior to the formation of the magnetic core, the coating, which is placed on the steel before the anneal, will be hardened by thefanneal. However, Where the grain growthanneal is provided after the formation.

Yof theY core, it will be apparent that the coating will not bein the hard form which is formed during the high temperature anneal. Therefore, in the formation of the core, the coating is more readily apt to yflake and break off during the handling'which is necessary in making Vthe magnetic core.

As hereinbefore noted, the coating which is generally applied to ferrous material inthe present stateY of .the art is a coating of magnesium hydroxide which -is applied in the form of a water slurry. The coating is then dried to leave a thin layer of coating material on 'theA surface of the ferrous material. In the present state of the art it Ais not, in general, possible to provide a satisfactory coat- -ing on the surface of a ferrous material using a substantially pure magnesium hydroxide slurry. The substan thickness on the ferrous material to withstand the subse-Y i quent handling and bendingrof the coated ferrous materiai, for example, the bending necessary in the formation of da magnetic core. The coating, under such circumstances, has ra great tendency to iiake and drop `off thus lcreating excessive dust during the formation of thecore with its resulting problems to equipment and to theV health of the workmen.v Of course, such akng also results'in i inferior interlaminar insulation after annealing.

A number of additives Vhave been proposed to be added to the magnesium hydroxide slurry which would help the magnesium hydroxide to adhere to the surface of the ferrous material. However, it has been found thatVmany of these additives create other problems. For example, many of these'additives' introduce additional carbon or other contaminants to the steel, thereby causing either higher initial lossesor higher aging losses, or both such additional losses, when such material is used in magnetic cores for electrical magnetic apparatus.V /From the above it is obvious that thereis aV great need in the electrical industry for a coatingrmaterial which will form a tenav cious, adherent coatingon ferrous material, such as silicon steel, while at the same time not detracting from `the optimum magnetic characteristics of such silicon steel.

It is, therefore, one object of this invention to provide a tenacious coating on ferrous material comprised substantially of magnesium hydroxide; which'coating will not detract lfrom the vmagnetic properties material.

Itis a further object of this invention to provide a process for coating ferrous material with a coating comprised substantially of magnesium hydroxide.

It is a further object of this invention to provide a magnesium hydroxide slurry which will firmly adhere to a ferrous material, in which the slurry is made from a special size of magnesium oxide.

It has recently been discovered that a substantially pure magnesium hydroxide coating can be applied to ferrous magnetic material when thefmagnesium hydroxide slurry is made` from magnesium oxidepowder having crystallites whose size falls Within a very narrow range. The crystallite size of the Vpowder is measured by X-ray diffraction .and is expressed `in angstrom units. Avery tenacious,

*Y of` the i ferrous adherent coating of substantially pure ,magnesiumhydroxide can be appliedto ferrous magnetic material when the crystallite size of the starting magnesium oxide powder falls in a range of approximately 170-208 angstrom units. As used throughout this specification' and claims, the term crystallite will ybe understood .to mean very small crystals; p Y Y In carrying out this invention in one form, an adherent lm of substantially pure magnesium hydroxide is pro- 'vided on the surface of la stripoffferrous 'material by lrst providing a slurry rcomprising a suspension of magnesium hydroxide in water. The slurry is formed from magnesiumy oxide powder haviniga crystallite size ranging from approximately 170 to 208angstrom units. Thesuspension is then applied lto the-surface of ,the ferrous- Ymaterial inany desired manner and then dried to remove any excess absorbed water. An `adherentilmfcomremain on the surface of the ferrous material.

prised substantially of 'pure magnesiumhydroxide will Y The features of this inventionV which are believed tobe novel .are set forth with rparti'cularity 'in the appended claims. However, it is believed that the invention itself andthe manner in which itsobjects are obtainedfaswell as other objects and advantages thereof, will be morefully understood by referenceto the following detailed descripthe range of approximately 170 to 208 angstrom units.

It has been found that a coating made from such slurry will very tenaciously .adhere toferrous magnetic material without necessity for any additive to the slur-ry to aid in adhering the coating to the ferrousmagneticmaterial.

Many attempts have'been madey to determine an acceptable characteristic or property of magnesium oxide powder which would provide an indication of its utility in formation of magnesium hydroxide coating. `It is generally well known that it is necessary to obtain substantially complete hydration of the powder inrwater suspensionto -obtain a slurry that will provide a good coating to ferrous magnetic material. However, many of the magnesium oxide powders that are substantially completely prepared, usingthe above magnesium oxide powder. vforming theaqueous suspension or slurry approximately 5.5 to 8% by Weightofthe magnesium oxide powder is 35 'placed in water and thoroughly mixed until substantially Y 4 1, l ide *to ferrous magnetic material. For example, some powders which were usedfin slurriesto provide magnesium hydroxide coatings had the following measured characteristics:

Powder No. Activity index Crystallite size, A.

AllV of the powders numbered l, 2 and 3V when made into Y lmagnesium hydroxide slurries formed magnesium hydrox-V In making the magnesium hydroxide coatings of this invention a magnesium oxidepowder having a crystallite size; approximately in the range of 170 to`208 angstrom units is employed.V In the preferred form of the invention, an aqueous suspension of magnesium hydroxide is In complete hydration ofthe magnesium oxide is obtained. This provides a slurry of magnesium hydroxide.

Ifa suspension of less than approximately'5.5% by Weight of magnesium oxide powder'is used tofornrthe slurry, theresultant coating does not provide avs'uflicient amount of magnesium hydroxide to the ferrous material. When the suspension contains more than approximately v8% by weight of magnesium oxider powder, the slurry becomes too difficult to process in the desired manner. If

powderihaving a crystallite size below approximately 170 .y angstrom units is used the adhesion of the coating to the LVroller'eoating on the material. This coating may be dried upon theV sheet at a surface temperature of not more than approximately 135? C. If the drying is carried out above Y this temperature, there vis a tendency for the coating to soV hydrated in a water suspension do not `provide a' good magnesium hydroxide coating.,

Some manufacturers of magnesium oxide powders have used an activity index as a proper powder characteristic vto classify various grades of V,magnesium oxide powder.

jIny general, activity index is a measurement of surface .area of the powder. However, this measurement of powder property does not necessarily bear fa direct relationship to the crystallite size of the powder, nor does, it

provide any indication'of the ability of the magnesium oxide powder to form a magnesium hydroxide slurry which will provide a good -coating of ,magnesium hydrox- I break down, leaving a spotty coating rather than the thin,

even coat desired. After drying there remains on the surface a thin film of substantially magnesium hydroxide which has an excellent resistance to abrasions and which will permit 90 bending without flaking'of` the coating thereon. Y For example, a coating using powder 3 of the above table prepared in a slurry, was applied to a strip of silicon steel 0.012 inch, thick. The coating obtained wask approximately 0.02 to 0.03 ounce per square foot of steel vand did not flake when subjected to 90 bending. After formation of a magnetic core from the silicon steel having been coated'by the'above suspension, the core may then be annealed, either a grain growth anneal in the range of approximately 950 to 1200,o C., or astress relief anneal of approximately 8407 C., without damage to the coating and without sticking or welding of the various layers of silifollowing table provides examples of the crystallite size of various magnesium oxide powders which were used to prepare magnesium hydroxide suspension. The slurries prepared from these powders were coated on silicon steel and dried, with the results indicated.

Table Crystallite size in angstrom units Powder No. Resulting coating Good cohesion. poor adhesion. Rubs) oi completely.

Do. Very good cohesion and adhesion. PoopD adhesion, fair cohesion.

Poor adhesion and cohesion.

No adhesion, fair cohesion.

No adhesion or cohesion.

Poor adhesion and cohesion. Vel-5;D good adhesion and cohesion.

Poor adhesion, fair coehesion.

Vers7 good adhesion and cohesion` No adhesion or cohesion.

Do. Do.

Do. Poor adhesion, fair cohesion. No adhesion or cohesion. Poor adhesion, fair cohesion. No adhesion or cohesion.

strongly coheres to form a tight coating on such material.

The scope of the invention set forth herein is defined in the appended claims.

What is claimed as new and which it is desired to secure by Letters Patent ofthe United States is:`

l. A ferrous material having thereon a rmly adherent coating consisting essentially of magnesium hydroxide formed by application of a slurry consisting essentially of an aqueous suspension of approximately 5.5 to 8% by Weight of a magnesium oxide powder, said powder having a crystallite size in the range of approximately 170 to 208 angstrom units, said coating being dried at a temperature not in excess of approximately C.

2. A method of forming an adherent coating on the surface of ferrous material consisting of the steps of forming a slurry consisting essentially of an aqueous suspension of approximately 5.5 to 8% by weight of magnesium oxide powder, having a crystallite size in the range of approximately to 208 angstrom units, applying said slurry to the surface ofthe ferrous material and then heating said surface to a temperature not in excess of approximately 135 C. to dry said slurry and leave on said surface a tenacious coating of magnesium hydroxide.

3. A ferrous material having on the surface thereof a firmly adherent coating of magnesium hydroxide formed by the application of a slurry consisting essentially of an aqueous suspension of a magnesium oxide powder having a crystallite size in the range of approximately 170 to 208 angstrom units, said coating being dried at a temperature not in excess of approximately 135 C.

4. A method of forming an adherent coating on the surface of a ferrous magnetic material consisting of the steps of forming a slurry consisting essentially of an aqueous suspension of magnesium oxide powder having a crystallite size in the range of approximately 17 0 to 208 angstrorn units, applying said suspension to the surface of the ferrous material, and drying said suspension on the surface at a temperature not in excess of yapproximately 135 C.

References Cited by the Examiner UNTED STATES PATENTS 2,909,454 10/59 Neish 117-127 RICHARD D. NEVIUS, Primary Examiner.

WILLIAM D. MARTIN, Examiner. 

1. A FERROUS MATERIAL HAVING THEREON A FIRMLY ADHERENT COATING CONSISTING ESSENTIALLY OF MAGNESIUM HYDROXIDE FORMED BY APPLICATION OF A SLURRY CONSISTING ESSENTIALLY OF AN AQUEOUS SUSPENSION OF APPROXIMATELY 5.5 TO 8% BY WEIGHT OF A MAGNESIUM OXIDE POWDER, SAID POWDER HAVING 